1. Field of the Invention
The present invention relates to an amplifier with feedforward (abbreviated hereinafter as FF) loops for rejecting non-linear distortion and control circuitry for optimizing FF loops which employs a method for compensating distortion generated in a main amplifier. The present invention particularly relates to a technique for compensating distortion such as intermodulation distortion generated in a main amplifier.
2. Description of Related Art
A base station or a relay station for mobile communications performs wireless transmission of a multicarrier signal including a number of modulated carriers. More specifically, a base station or a relay station modulates each of the multiple carriers and arranges those carriers on a frequency axis at a certain frequency separation to obtain a multicarrier signal. The base station or the relay station then executes radio-frequency amplification (RF amplification) of the obtained multicarrier signal, and performs wireless transmission of the multicarrier signal after the RF amplification. Accordingly, such a station requires an amplifier for executing RF amplification of a multicarrier signal. Further, in order to favorably communicate with a mobile station located within a coverage or a cell, the station typically requires a RF amplifier that can perform high power amplification. A similar need also exists in a booster or like devices.
In an amplifier used for amplifying a multicarrier signal, superior linearity is required over the entire frequency range to which the multicarrier signal belongs because, if the linearity of the amplifier is not sufficient, normal and high-quality communication would be obstructed by distortion generated in the amplifier. A variety of distortions exist that are caused by the non-linearity of the amplifier. Among those distortions, distortion being produced at a frequency identical to or extremely close to that of a carrier like IMD (intermodulation distortion) cannot be, or remains very difficult to be, eliminated by an approach such as providing a filter after the amplifier. Nevertheless, distortions having such nature are likely to occur when amplifying a multicarrier signal.
One approach for providing an amplifier having an extremely low amount of distortion which is suitable for amplifying a multicarrier signal is to improve the linearity of the amplifier by adding circuitry to the amplifier. One known example technique of such an approach is the FF amplification method disclosed in Japanese Patent Laid-Open Publication No. Hei 4-70203. An amplifier adopting the FF amplification method comprises a distortion detection loop and a distortion rejection loop.
The signal path from the signal input terminal to the signal output terminal passing through the main amplifier, that is, the signal path for transmitting the input signal into the main amplifier to be amplified and the signal amplified by the main amplifier, is referred to as the dominant path. To simplify notation in the present application, the signal transmitted in the dominant path is hereinafter referred to as the dominant signal. The signal passing through the dominant path before the main amplifier is referred to as the input signal. The signal passing through the dominant path from the output terminal of the main amplifier to the point of being subjected to distortion compensation is referred to as the output signal. The signal passing through the dominant path after the point of distortion compensation is referred to as the distortion-compensated output signal.
The distortion detection loop provides as a feedforward a first branch signal obtained by branching a portion of the input signal at a first branching point to a first coupling point located thereafter. At a second branching point located after the first branching point and a main amplifier, a portion of the output signal is branched as a second branch signal. The second branch signal and the first branch signal provided as a feedforward are combined at the first coupling point.
The input signal and the first branch signal branched therefrom include a plurality of carrier components constituting the multicarrier signal, but do not, at any time, include distortion components generated in the main amplifier or its surrounding circuitry (hereinafter collectively referred to as "the main amplifier"). On the other hand, when distortion components are being generated in the main amplifier, the output signal and the second branch signal branched therefrom include both the carrier components and the distortion components. Accordingly, when combining the first and the second branch signals, if the first and the second branch signals to be combined are in a relationship such that their respective carrier components cancel each other out, a signal including only the distortion components can be obtained. A signal obtained as such is hereinafter referred to as the distortion signal.
To obtain a highly pure distortion signal having only the distortion components, the first and the second branch signals must be in a relationship such that their respective carrier components completely cancel each other. Specifically, a first requirement for this relationship is that the electrical wave length of the signal path from the first branching point to the first coupling point passing through the distortion detection loop must be identical with the electrical wave length of the signal path from the first branching point to the first coupling point passing through the main amplifier and the second branching point. A second requirement is that, at the first coupling point, the first and the second branch signals must have an identical amplitude and an opposite phase from one another.
The distortion rejection loop provides the distortion signal as a feedforward to be recombined with the output signal at a second coupling point located after the first and the second branching points. If the signal delay occurring in the distortion rejection loop is compensated in the dominant path, and if the distortion components in the output signal and the distortion signal through the auxiliary amplifier are appropriately adjusted in the distortion rejection loop or in the dominant path such that their respective amplitudes are identical and their phases are opposite from one another, the signal recombining operation at the second coupling point rejects distortion components generated in the main amplifier to provide a distortion-compensated output signal having no, or a suppressed amount of, distortion components.
FIG. 8 shows an example configuration of a conventional FF amplifier. In this amplifier, three hybrids HYB1-HYB3 are used to form the distortion detection loop L1 and the distortion rejection loop L2. In the Figure, the signal path from the signal input terminal IN to the signal output terminal OUT passing through the main amplifier A1 and the coaxial delay line D2 is the dominant path. The signal path from the first branching point inside hybrid HYB1 to the first coupling point inside hybrid HYB2 passing through the coaxial delay line D1 is the distortion detection loop L1. The signal path from the first coupling point to the second coupling point inside hybrid HYB3 passing through the auxiliary amplifier (distortion amplifier) A2 is the distortion rejection loop L2. Respective dummy loads Z0 in the Figure have an impedance equal to the characteristic impedance of the transmission line, and is used as the termination for hybrids HYB1 and HYB3 terminals. The second branching point is located inside hybrid HYB2.
The signal applied to the signal input terminal IN, namely, the input signal, is a multicarrier signal, for example. This signal is input, via hybrid HYB1, into variable attenuator ATT1 and variable phase shifter PS1. After being subjected to amplitude and phase adjustment therein, the input signal is amplified by the main amplifier A1. The signal amplified by the main amplifier A1, namely, the output signal, is input into hybrid HYB3 via hybrid HYB2 and the coaxial delay line D2. Further, the distortion-compensated output signal is output from hybrid HYB3 to subsequent circuitry via the signal output terminal OUT. The coaxial delay line D2 is the delay line for compensating the delay exerted on the distortion signal by the circuitry that constitute the distortion rejection loop L2 including the auxiliary amplifier A2.
Furthermore, the input signal is branched into two signals by hybrid HYB1. The two branched signals are identical signals with respect to frequency structure of their components. One of the two branched signals which is to be provided in the dominant path is supplied to the main amplifier A1 as the input signal and is thereby amplified. The other of the two branched signals which is to be provided in the distortion detection loop L1, namely, the first branch signal, is supplied from hybrid HYB1 to hybrid HYB2 via the coaxial delay line D1 while its amplitude is mostly maintained as is. The coaxial delay line D1 is the delay line for compensating the delay exerted on the dominant signal by the circuitry of the dominant path, especially the main amplifier A1.
At the second branching point located therein, hybrid HYB2 branches into two signals the signal output from the main amplifier A1 including distortion components. The two branched signals are identical signals with respect to frequency structure of the components. One of the two branched signals is supplied to the dominant path as the output signal. The other of the two branched signals, namely, the second branch signal, is combined with the first branch signal in the first coupling point inside hybrid HYB2. If the distortion detection loop L1 is optimized as described below, this combining operation in hybrid HYB2 cancels the carrier components in the first and the second branch signals, producing the distortion signal indicating the distortion components generated in the main amplifier A1.
The distortion signal obtained in this way is supplied from hybrid HYB2 sequentially to variable attenuator ATT2, variable phase shifter PS2, and the auxiliary amplifier A2, constituting the distortion rejection loop L2. Specifically, the distortion signal is subjected to amplitude and phase adjustment in variable attenuator ATT2 and variable phase shifter PS2, amplified by the auxiliary amplifier A2, and input into hybrid HYB3. The distortion signal input into hybrid HYB3 is combined in the second coupling point inside hybrid HYB3 with the dominant signal transmitted via the coaxial delay line D2. If both of the distortion detection loop L1 and the distortion rejection loop L2 are optimized as described below, this combining operation in hybrid HYB3 produces the distortion-compensated output signal in which the distortion components are eliminated or suppressed (by canceling out). The distortion-compensated output signal is output from the signal output terminal OUT.
To generate a highly pure distortion signal by combining the first and the second branch signals and canceling out the carrier components, a predetermined number of carrier components contained in respective ones of the first and the second branch signals must have an identical timing, an identical amplitude, and an opposite phase from one another at the first coupling point. To fulfill these requirements, that is, to optimize the distortion detection loop L1, the circuit shown in FIG. 8 is provided with the coaxial delay line D1 as means for providing an identical timing to the respective carrier components, and variable attenuator ATT1, variable phase shifter PS1, and the control circuit 10 as means for providing an identical amplitude and an opposite phase to the respective carrier components. The control circuit 1 is the means for adjusting the output of hybrid HYB2 such that a distortion signal including primarily only the distortion components and no carrier components is supplied to the auxiliary amplifier A2. This adjustment is performed in the control circuit 10 by adjusting and controlling the signal attenuation G1 and the phase shift .theta.1 in variable attenuator ATT1 and variable phase shifter PS1 to their respective optimal values.
To generate a favorable distortion-compensated output signal through combining the output signal and the distortion signal, it is preferred that the distortion signal transmitted via the auxiliary amplifier A2 primarily includes no carrier components but only the distortion components. This can be fulfilled by optimizing the distortion detection loop L1, because, as long as the distortion detection loop L1 is operating normally, distortion generating in the auxiliary amplifier A2 can be ignored. The second requirement in compensating distortion generated in the main amplifier A1 is that, at the second coupling point, the distortion components in the output signal transmitted via the coaxial delay line D2 and those in the distortion signal via the auxiliary amplifier A2 must have an identical timing, an identical amplitude, and an opposite phase with respect to one another. To fulfill this second requirement, that is, to optimize the distortion rejection loop L2, the circuit shown in FIG. 8 is provided with the coaxial delay line D2 as means for providing an identical timing to the distortion components in respective signals, and variable attenuator ATT2, variable phase shifter PS2, and the control circuit 10 as means for providing an identical amplitude and an opposite phase to the distortion components in respective signals. The control circuit 10 adjusts and controls the signal attenuation G2 and the phase shift .theta.2 in variable attenuator ATT2 and variable phase shifter PS2 to their respective optimal values to generate a distortion-compensated output signal in which the distortion components are eliminated or suppressed.
The control circuit 10 executes the adjustment and control of the above-mentioned G1, .theta.1, G2, and .theta.2 to their optimal values, which are the processes for optimizing the distortion detection loop L1 and the distortion rejection loop L2. In FIG. 8, these optimizing processes are executed by the control circuit 10 through insertion and detection of two kinds of pilot signals under the control of the CPU.
The control circuit 10 comprises oscillators OSC1 and OSC2, and is connected with directional couplers DC1-DC4. Oscillators OSC1 and OSC2 generate the pilot signals for L1 and L2, respectively.
Directional coupler DC1 connected to oscillator OSC1 is disposed before the first branching point located inside hybrid HYB1, so as to insert the pilot signal for L1 into the input signal and the first branch signal branched therefrom. Directional coupler DC2 is disposed along the path between the first coupling point inside hybrid HYB2 and the second coupling point inside hybrid HYB3 passing through the auxiliary amplifier A2, so as to detect the presence of the pilot signal for L1 and its level in the distortion signal.
Directional coupler DC3 connected to oscillator OSC2 is disposed along the path between the first branching point inside hybrid HYB1 and the second branching point inside hybrid HYB2 passing through the main amplifier A1 (may be inside the main amplifier A1), so as to insert the pilot signal for L2 into the output signal and the second branch signal branched therefrom. Directional coupler DC4 is disposed between the second coupling point inside hybrid HYB3 and the signal output terminal OUT, so as to detect the presence of the pilot signal for L2 and its level in the distortion-compensated output signal.
The control circuit 10 inserts or superimposes the pilot signal for L1 in the input signal using directional coupler DC1, and detects the pilot signal for L1 using directional coupler DC2. The control circuit 10 controls the signal attenuation G1 and the phase shift .theta.1 such that the detected level of the pilot signal for L1 becomes lower in directional coupler DC2, thereby optimizing the distortion detection loop L1. In other words, the signal attenuation G1 and the phase shift .theta.1 are controlled such that the pilot signal for L1 does not appear in the distortion signal.
Further, the control circuit 10 inserts or superimposes the pilot signal for L2 in the output signal using directional coupler DC3 before the second branching point, and detects the pilot signal for L2 using directional coupler DC4. The control circuit 10 then controls the signal attenuation G2 and the phase shift .theta.2 such that the detected level of the pilot signal for L2 becomes lower in directional coupler DC4, thereby optimizing the distortion rejection loop L2. In other words, the signal attenuation G2 and the phase shift .theta.2 are controlled such that the pilot signal for L2 does not appear in the distortion-compensated output signal.
Processes for determining G1, .theta.1, G2, and .theta.2 are primarily executed by the CPU 12 and the control signal generator 14 inside the control circuit 10.
Out-of-band undesired signals are first eliminated from the signals detected in directional couplers DC2 and DC4 using band-pass filters BPF1 and BPF2, respectively. To further facilitate signal handling, these signals are then mixed with an oscillated output of the local oscillator LOC using mixers MIX1 and MIX2. From among the resulting signals, low-pass filters LPF1 and LPF2 extract the difference frequency components, namely, the signals converted to a lower frequency than original. The extracted components are input into the control signal generator 14 via amplifiers or buffers B1 and B2. The control signal generator 14 generates control signals related to G1, .theta.1, G2, and .theta.2 under the control of the CPU 12 following a step-by-step logic and method. The step-by-step method herein refers to a repeated sequential execution of the process of slightly shifting the values of the control signals in an arbitrary direction to search for the direction of change toward which the output levels from amplifiers or buffers B1 and B2 would be lower, and changing the control signal values in that direction.
Although an amplifier having an extremely low amount of distortion suitable for amplification of a multicarrier signal can be formed according to the above-described circuit arrangement, several problems still remain.
When there are changes in, for example, the level of the input signal, the number of carriers, and temperature level, operating conditions are altered in the main amplifier A1 and the auxiliary amplifier A2. If the control signals are generated by the step-by-step process as described above, it is difficult to follow rapidly such an alteration in operating conditions of the main amplifier A1 and the auxiliary amplifier A2 upon its occurrence. In other words, the time it takes for the loops to balance under the new operating conditions after alterations and for the detected levels of the pilot signals to accordingly settle close to zero, namely, the acquisition time of the loops with respect to alterations in operating conditions, becomes long such that it cannot in practice be ignored.
Especially, if the above-described conventional technique is used in the above-mentioned field of RF amplifiers for transmission in a base station for mobile communications, for example, the acquisition time of the loops with respect to alterations in operating conditions may be as long as 3 to 10 seconds. Moreover, the auxiliary amplifier A2 may receive excessive input during the time period from the occurrence of an operating condition alteration to the balancing of the distortion detection loop L1, and, when such state is notable, the auxiliary amplifier A2 may become damaged.
As the pilot signal for L1 is included in the output signal transmitted via the coaxial delay line D2, the pilot signal for L1 undesirably remains in the distortion-compensated output signal. The residual pilot signal for L1 may become an impediment to operation in subsequent circuitry. For example, in the application of RF amplifier for transmission in a base station for mobile communications, undesirable spurious effect is caused when the distortion-compensated output signal having residual pilot signal for L1 is supplied as is to an antenna.
To prevent such undesirable effects by using additional circuitry in the circuit of FIG. 8, for example, a notch filter for blocking the pilot signal for L1 may be disposed in a section after the second branching point inside hybrid HYB2 along the dominant path. Alternatively, a circuit may be provided for injecting into the dominant path a signal that cancels out the pilot signal for L1. However, as the notch filter would filter the signal amplified by the main amplifier A1 having high power, a large and expensive notch filter must be used. In addition, disposing a notch filter would cause degradation in phase linearity of the entire circuit. Generation of insertion loss by the notch filter would also lower the operating efficiency of the entire circuit. A circuit for injecting into the dominant path a signal that cancels out the pilot signal for L1, on the other hand, is not practical because its structure would be complex and the control for temperature compensation or the like would be difficult.